1. Field of the Invention
The present invention relates to an adjusting method for centering an earth electrode at an axis of a center electrode of a spark gap of a spark plug for an internal combustion engine mounted on a automotive vehicle.
2. Description of the Related Art
In conventional spark plugs, a columnar center electrode is fixed and electrically isolated in the spark plug housing, while one end of the earth electrode is welded to the housing and the other end of the earth electrode is bent perpendicular to the tip surface of the center electrode, thereby forming a spark gap.
The spark gap length is adjusted within a prescribed tolerance and further a positional difference (measured along a direction perpendicular to the axis of the center electrode) between the axis of the center electrode and the center of the end surface of the earth electrode is also adjusted within other prescribed tolerance. It is ideal that the off-axis positional difference is zero.
Although, for example, JP2000-329529A discloses a method for observing the spark gap, wherein an edge image of the center electrode and earth electrode is obtained by an oblique illumination surrounding them and the image is processed, there is not yet in the field of the image processing any established method for measuring the above-mentioned off-axis positional difference between the center electrode and earth electrode. Thus, the off-axis positional difference could not be efficiently adjusted.